1. Field of the Invention
The present invention relates to a plastic molded type power semiconductor device, especially used for controlling a high-voltage power, in which a semiconductor chip is mounted and set on a heat dissipation member of lead frame, the heat dissipation member is covered with an external plastic material so as to also cover its lower surface, and the semiconductor device is used while being mounted on a mounting plate having a heat dissipation function.
2. Description of the Related Art
Since a power control semiconductor device generates heat during an operation of the device, an important condition in its arrangement is that heat dissipation characteristics of the device are set to be sufficient. In a plastic molded type power semiconductor device, a heat dissipation section incorporated in the semiconductor device which is tightly mounted on an external mounting member constituting a heat dissipation unit having, e.g., a cooling fin or the like, is used. In this case, the external mounting member must be made of a material having good heat conductivity. For example, as the material, a metal material such as aluminum generally having electric conductivity is used.
FIG. 6 is a conventional plastic molded type power semiconductor device. The semiconductor device comprises a heat dissipation member 41 made of a metal material such as copper, and a power control semiconductor chip 43 is mounted on the heat dissipation member 41 by a Pb-Sn solder 42. In this case, the heat dissipation member 41 also serves as a heat dissipation plate for conducting heat generated from the semiconductor chip 43. In addition, a lead member 44 made of the same material as that of the heat dissipation member 41 is set at a position spaced apart from the heat dissipation member 41. The lead member 44 is connected to an extending terminal of the semiconductor chip 43 on the heat dissipation member 41 by a bonding wire 45. Thus, a main body of the semiconductor device is constituted.
The main body constituted by the heat dissipation member 41, the semiconductor chip 43, the lead member 44, and the bonding wire 45 is sealed by a transfer-molded plastic body 46. In this case, an insulated type structure in which the heat dissipation member 41 is covered with the plastic body 46 so as to also cover its lower surface portion is used, and the heat dissipation member 41 is mounted by screws or the like on an external mounting member 47 made of a metal material through the plastic layer on the lower surface of the heat dissipation member 41.
In the semiconductor device arranged as described above, even when the device is directly mounted on the external mounting member 47 to dissipate heat, since the external mounting member 47 is electrically insulated from the heat dissipation member 41, an insulating member such as a mica plate need not be used. Therefore, the plastic molded type semiconductor device can be practically used with good workability, and this structure has been used as a major structure of a power semiconductor device. However, in accordance with a decrease in size of a semiconductor device of this type, various problems are posed. For example, when a semiconductor device sealed by a sealing plastic is mounted on the external mounting member 47, an interval a between an outer lead 441 portion extending from the plastic body 46 of the lead member 44 to the outside and the mounting member 47 has a minimum size determined by the rated dielectric breakdown voltage. In order to decrease the size of the semiconductor device, the thickness of the insulator on the lower surface portion of the heat dissipation member must be decreased as much as possible. However, when the thickness is decreased, the interval a is naturally decreased, and a satisfactory dielectric breakdown voltage cannot be obtained. For this reason, as shown in FIG. 6, the height of the heat dissipation member 41 from the external mounting member 47 is different from that of the lead member 44 to position the lead member at a level higher than the major surface of the heat dissipation member 41.
When the heat dissipation member 41 and the lead member 44 are arranged to have a step difference therebetween, the structure of the member portion is complicated, and a connecting operation of the bonding wire 45 is cumbersome. In addition, since the semiconductor chip 43 on the heat dissipation member 1 is connected to the lead member 44 by the bonding wire 45, a ratio of a size (thickness) above a major surface of the heat dissipation member 41 to a size (thickness) below the major surface of the heat dissipation member 41 is usually "(5 to 10) : 1". Therefore, a thickness b of the semiconductor device is limited, and a decrease in size of the semiconductor device is limited.